1. Field of the Invention
The invention relates to the measurement of surface features of test objects using frequency-shifting interferometry, and in particular to such measurement of test objects having distinguishable surface regions that may be subject to differing measuring requirements.
2. Description of Related Art
Frequency-shifting interferometry, which involves the collection of interference data at multiple measuring beam frequencies for converting the accompanying variations in intensity into a measure of optical path length difference between interfering portions of the measuring beam, is noted for the capability of taking measurements over a wider range of distances than conventional phase-shifting interferometry. For example, test surfaces having discontinuities well beyond the measuring beam wavelength can be unambiguously measured using frequency-shifting interferometry.
However, considerable pixel-by-pixel processing is required to complete the measurement of test surfaces over which frequency-shifting interference data is gathered. Each pixel collects light from a finite area of the test surface, and separate calculations are carried out for each pixel of recorded interference data for measuring local height variations of the test surface (i.e., surface topographies). The calculations convert the intensity data collected for each pixel at the multiple measuring beam frequencies into a measure of surface topography, and the calculations for each pixel take time.
U.S. Pat. No. 6,741,361 entitled MULTI-STAGE DATA PROCESSING FOR FREQUENCY-SCANNING INTERFEROMETER discloses examples of the pixel-by-pixel processing in multiple stages. Increased accuracy is achieved by additional stages of processing. Although the disclosed multi-stage processing saves time, the additional processing for achieving increased accuracy takes more time than measurements of lesser accuracy.
Co-assigned U.S. application entitled PHASE-RESOLVED MEASUREMENT FOR FREQUENCY-SHIFTING INTERFEROMETER, filed on even date herewith, discloses further improved processing procedures for achieving high accuracies typical of conventional phase-shifting interferometers over a range of measurement typical of phase-shifting interferometers or beyond. Each pixel is calculated separately based on intensity measurements acquired at multiple measuring beam frequencies.
Computer processing time associated with the measurement of surface topographies, particularly when higher accuracies are required, can add considerably to the overall time required to measure the surfaces. The time for taking measurements is particularly important in the manufacturing environment where expensive operations can be slowed by the measurements. To avoid production down time, inspection criteria can be lowered, fewer test objects can be inspected, or less satisfactory inspection techniques can be applied.
The surfaces of some test objects have different regions over which different measuring criteria can be applied. For example, some regions of test surfaces can require measurement to higher accuracy than other regions of the same test surfaces, including different regions imaged by the same interference patterns. Generally, the interference patterns are evaluated as a whole or within more limited areas that are within a range of measurement.
Frequency-shifting interferometers are capable of measuring rougher surfaces and surfaces with larger discontinuities than conventional phase-shifting interferometers, including surfaces with regular variations that exceed one fringe spacing (e.g., one-half of the measuring beam wavelength). The interference patterns evaluated by frequency-shifting interferometers are often apparent as speckle patterns having no discernable fringes and extending largely without interruption over a wide range of surface variations.